This invention relates to improvements in impact printers of the type generally employed in data processing systems and more particularly to a drum printer particularly configured to achieve superior vertical registration of printed characters.
Impact printers employing rotating character drums are well known in the prior art. They generally include a plurality of hammers mounted with their impact faces aligned in a row extending parallel to the drum axis. As the drum rotates, different rows of characters on the drum are successively moved into the print station; i.e. into a position opposed to the row of impact faces. Actuation of a hammer propels it against the opposed character on the drum to thus print the character on a paper web therebetween.
The characters on the drum are conventionally arranged in rows extending parallel to the drum axis and rings extending around the drum axis. Typically, the printer will include a number of hammers equal to the number of rings on the drum so as to enable a full line of text to be printed during a single drum revolution. After each line is printed, the paper web is moved by a unit length corresponding to the line spacing, and a subsequent line is printed.
Since the characters on the drum move past the hammer impact faces perpendicular to the lines of text, slight timing variations between the impacting of the hammers results in vertical misalignment of characters in the printed line. Such misalignments, of course, become more pronounced as printing speeds increase.
A different type of impact printer which avoids the aforementioned vertical misalignment problem utilizes a type bearing surface, such as a chain or band, in lieu of the drum, which moves characters past the hammer impact faces along a line of text. In this type of printer, impact timing variations result in horizontal print variations which are not nearly as noticeable to the eye as are vertical misalignments. The disadvantages of known printers using such horizontally moving characters has been primarily that they are more costly and require more maintenance than known drum type machines.